The present invention relates generally to the fields of radar systems and to output frequency varying magnetrons and more specifically to the fields of inverse synthetic aperture radars and to stepped frequency radar transmitters.
The frequency agile magnetron imaging radar of the present invention can be used for generating range profiles and inverse synthetic aperture radar images from the range/Doppler characteristics of the target as measured using a stepped frequency waveform implemented for magnetron radars. The stepped frequency concept itself is a coherent radar technique described in U.S. patent application Ser. No. 268,501 referred to above. Briefly, the target range/Doppler behavior is obtained from the echoes produced by the target from a series of N "bursts" of n pulses. The N echo bursts are received over a period of time T as the target's aspect changes. Each burst contains a series of n pulses, each pulse being stepped .delta.f in frequency from the previous pulse. Echoes from each of the n pulses during a burst are sampled to obtain n complex pulses of the target, each at a different frequency. The inverse discrete Fourier transform of the series of n pulses of each burst is a "synthetic" range profile of the target. N bursts produce N range profiles. Aspect motion of the target relative to the radar during the collection of the N synthetic range profiles produces a Doppler frequency spread related to cross-range position of individual target scatterers. Doppler spectrum is measured for each range cell by a series of Fourier transforms, but now in the time history dimension for each range cell of the N bursts. The resulting n.times.N range/Doppler matrix is the inverse synthetic aperture radar image of the target. All signals are derived from a stable oscillator using direct frequency synthesis techniques.
A primary problem in the implementation of the inverse synthetic aperture radar concept described above as well as in the implementation of the 3D imaging radar system described in U.S. patent application Ser. No. 300,344 referred to above is the requirement for utilization of a coherent power amplifier to transmit the precise stepped frequency radar pulses. Existing radar systems may not include coherent power amplifier transmitters. Further, such amplifiers are very expensive when compared with the cost of magnetrons already in use in many radar systems.